JPC SYSTEMIC PATHOLOGY
MUSCULOSKELETAL SYSTEM
March 2022
M-M13 (NP)

Signalment (JPC #75-10):  Neonatal Chester white piglet

 

HISTORY:  Six littermates of this piglet were stillborn and 6 more died within a few hours of birth.  Remaining piglets, including this one, were unable to stand or adduct their hind legs, which extended forward and laterally (“splay legged”).  Foreleg involvement was also seen in some piglets.  Gross lesions were not evident at necropsy.

 

MICROSCOPIC DESCRIPTION:  Skeletal muscle:  Diffusely myofibers are small and irregularly sized, and contain decreased numbers of myofibrils that are aggregated centrally or eccentrically and surrounded by clear space.  Diffusely, myocytes are pale, lack cross-striations, and frequently have large, vesiculate nuclei.

 

MORPHOLOGIC DIAGNOSIS:  Skeletal muscle:  Myofibrillar hypoplasia, diffuse, moderate, Chester white, porcine.

 

CONDITION:  Myofibrillar hypoplasia

 

SYNONYMS:  Splayleg, spraddle-leg

 

GENERAL DISCUSSION:

• Congenital disorder of neonatal piglets, resulting in lateral splaying of legs (inability to adduct)
• Transient effect on locomotion with survivors normalizing within two weeks
• Incidence fluctuates, but frequently appears as a farm or regional outbreak
• Disease incidence within litters is variable; may occur in consecutive litters

 

PATHOGENESIS:

• Unknown; proposed but unproven etiologies include: genetic predisposition, infectious agents, nutritional causes, mycotoxins and potassium-sodium imbalances
• Most prevalent in neonatal Landrace and other large white breed piglets
• Males predisposed
• Usually affects the semitendinosus, longissimus dorsi, and triceps muscles
• Skeletal muscle development in nomal piglets: In utero skeletal muscle development occurs by a “stem line-template” method
  • A large, type 1 muscle fiber (slow twitch, rich in myoglobin) centrally located within a sub-lobule divides longitudinally to create a smaller daughter fiber with type 2 myofiber characteristics (fast twitch, rich in glycogen) or serves as a template for myoblasts to evolve new fibers
  • The central template fiber continues to spawn new fibers and push previously formed fibers to the periphery forming a sublobule of 10-20 fibers
  • About the time of birth, the new type 2 fibers change to a type 1 fibers and the original central fiber ceases dividing
• Skeletal muscle development in splayleg piglets:
  • About the time of birth, the transition of type 2 fibers to type 1 fibers is delayed (suggests delayed maturation of muscle in utero)
  • Postnatal maturation continues, and the fibers complete the transition by day 7-8; at this time there is rapid increase in muscle fiber size, and clinical signs disappear
  • As piglets recover, their muscles appear similar to those of normal neonates

 

TYPICAL CLINICAL FINDINGS:

• Signs, characterized primarily by muscle weakness, are present at birth or by 2-3 hours after birth
• Hindlegs or all four legs are laterally extended
• Pigs are unable to adduct their limbs, they propel by pushing against the ground using pelvic limbs
• Animals are mentally alert and otherwise normal
• Spontaneously resolves in 1-2 weeks if animal survives

 

TYPICAL GROSS FINDINGS:

• The muscles typically involved include the longissimus, semitendinosus, and triceps groups
• Affected muscles are flabby, soft, and pale
• All four legs extend out
• Flattened sternum due to propelling against ground

 

TYPICAL MICROSCOPIC FINDINGS:

• Myofibers have a reduced diameter
• Individual muscle fibers have an abnormally small mass of myofibrils resulting in pale staining with eosin
• The remainder of the muscle fiber sheath has large clear spaces (glycogen) and often contains eosinophilic proteinaceous fluid

• PAS staining reveals cytoplasmic glycogen staining

• Occasional fibers may exhibit segmental myolysis
• These changes may occur in some piglets without clinical splayleg

 

ULTRASTRUCTURE:

• Z-line streaming: Segmental Z-line distortion with dispersal of electron-dense material
• Loss of register and alignment of sarcomeres and thinning of myofibrils
• Myofilament bundles fill only part of the cytoplasm in the muscle cell, are occasionally split, and have irregular cross striations
• The remaining space is filled with granular material consisting of glycogen, ribosomes, and lysosomes

 

DIFFERENTIAL DIAGNOSIS:

• Splayleg conditions (without myofibrillar hypoplasia)
  • Congenital spinal abnormalities: Dysraphism, arthrogryposis, hydromyelia, syringomyelia, and other abnormalities of the dorsal, central, or ventral gray matter and ventral median fissure (reduced myelination of the lumbar spinal may play a role)
  • Iron injection myopathy

 

COMPARATIVE PATHOLOGY:

• Ruminants:
  • Cattle: Myofibrillar hypoplasia reported in a 6 week old calf with similar lesions to those in pigs
  • Skeletal muscle hypoplasia represents the only significant lesion in peripheral organs of ruminants infected with Schmallenberg virus during gestation
  • Hypoplastic muscular changes are a characteristic finding in lambs and calves after natural or experimental infections with Orthobunyaviruses, as well as in hamsters and chicken embryos infected experimentally with Akabane virus
• Dogs: Similar syndrome described in puppies (“swimmer pups”) but often resolves with specific clinical therapy

 

REFERENCES:

1. Cooper BJ, Valentine BA. Muscle and Tendon. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St Louis, MO: Elsevier; 2016: 189-190.
2. Dewey CE. Diseases of the nervous and locomotor systems. In: Straw BE, Zimmerman JJ, D'Allaire S, Taylor DJ, eds. Diseases of Swine. 9th ed. Ames, IA: Blackwell Publishing; 2006:88.
3. Olson EJ, Carlson CS. Bones, Joints, Tendons, and Ligaments. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:945.
4. Radostits OM, Gay CC, Blood DC, Hinchcliff KW. Veterinary Medicine, a Textbook of the Diseases of Cattle, Sheep, Pigs, Goats and Horses. 9th ed. Philadelphia, PA: W B Saunders; 2000:1800-1801.
5. Seehusen F, Hahn K, Herder V, Weigand M, Habierski A, Gerhauser I, Wohlsein P, Peters M, Varela M, Palmarini M, Baumgärtner W. Skeletal muscle hypoplasia represents the only significant lesion in peripheral organs of ruminants infected with Schmallenberg virus during gestation. J Comp Pathol. 2014; 151(2-3):148-52.

Click the slide to view.

---